Thermoplastics (TPs) are very important items of commerce. Typically they are formed into various parts and shapes by melt forming, that is melting of the TP, forming it while molten into a shape and then cooling the TP to a solid to “fix” it in that shape. In most melt forming machines, the TP is fed in the form of a pellet or granule, typically in the size range of 0.1 to about 0.7 cm (longest dimension). In order for most melt forming machines to work efficiently, it is preferred that the pellets or granules be free flowing and have a reasonably uniform size.
Many types of apparatuses have been developed to pelletize TPs. Such an apparatus should preferably produce uniform and readily flowing pellets, at low cost. One such type of pelletizing apparatus is the so-called “underwater melt cutter” (UMC), see for instance U.S. Pat. Nos. 2,918,701 and 3,749,539. When a UMC is operating properly, it is capable of producing large amounts of TP pellets which are uniform and free flowing. However, UMCs have a number of drawbacks, among these difficulty in pelletizing higher melting point (>200° C.) TPs or TPs that otherwise readily freeze to solids, intolerance to process upsets such as short interruptions in polymer flows, and sometimes difficult startups. Thus improvements that would minimize these and other difficulties with UMCs are desired.
U.S. Pat. No. 4,728,276 describes an “Underwater Pelletizer” with die holes having what appear to be a reverse taper. No mention is made of the effect of such die holes on a startup.
Japanese Patent Application 5-253997 describes a die whose holes have a reverse taper. The purpose of these holes appears to be minimization of die drips and their degradation in cutters which are not UMCs.
It is therefore an object of the invention to provide a die useful in an underwater melt cutter that minimizes various difficulties associated with the start up of UMC's. These and other objects, features and advantages of the invention as disclosed and claimed herein will become apparent upon having reference to the following detailed description of the invention.